The Importance of Water
Biology: IB Diploma Higher Level, topic 1.1.1
Function of Water
Water makes up 60-70% of the human body. Life also evolved in a watery environment. Water's chemical structure gives it properties that are vital for life.
Uses of water
- Water is one of the most useful molecules for life.
- Water acts as a reactant in cells, for example in photosynthesis and hydrolysis reactions.
- Water provides structural support in cells, for example by keeping them turgid.
- Water helps keep organisms cool, maintaining body temperature.
Hydrogen bonding in water
- Water molecules are polar, with a slightly negative oxygen atom and slightly positive hydrogen atoms.
- A polar molecule has an unequal spread of charge.
- The charges allow water molecules to form hydrogen bonds with each other.
- Hydrogen bonding gives water properties that are vital for life.
Cohesion and adhesion
- Hydrogen bonds cause cohesion in water.
- Cohesion is a pull between water molecules.
- Cohesion pulls water up through xylem vessels in plants.
- Water also shows adhesion, a pull between water and other polar surfaces.
- Adhesion helps water stick to the walls of xylem vessels.
Thermal properties
- Water has a high specific heat capacity.
- A high specific heat capacity means water needs lots of energy to warm up.
- High specific heat capacity keeps aquatic habitats at a stable temperature.
- Water has a high latent heat of vaporisation.
- Evaporation of sweat cools the body down.
Water as a solvent
- Water is a good solvent.
- Water's polarity attracts charged and polar substances.
- Hydrophilic substances are polar or ionic and dissolve easily in water.
- Hydrophobic substances are non-polar and do not dissolve in water.
- Water carries dissolved substances around the body.
Density of water
- Ice is less dense than liquid water.
- Hydrogen bonds hold the molecules further apart in ice than in liquid water.
- Lower density means ice floats on top of liquid water.
- Floating ice insulates the water below, protecting aquatic organisms from freezing.
Test yourself on The Importance of Water
Decide which statement about water's properties is correct in each pair.
What % of the human body is made up of water?
What would happen to water transport in xylem vessels if hydrogen bonds between water molecules were much weaker?